The invention relates to unstable optical resonators and, more particularly, to an improvement to a mirror thereof, whereby interfering edge waves travelling from the mirror toward the geometric source point of the resonator cancel the edge wave from the output mirror of the resonator.
Unstable optical resonators offer higher extraction efficiency combined with good beam quality. They are distinguished by an output that expands around rather than passing through the output mirror. Diffractive scattering back into the resonator from the sharp aperture at the edge of the output mirror significantly affects the mode structure of such resonators. An analysis which neglects edge diffraction yields a lowest-order mode that has uniform intensity and is substantially favored over higher-order modes. The effect of edge diffraction is to add structure to the amplitudes of the modes and to change their relative feedback ratios. Mode crossing of the dominant mode as a function of equivalent Fresnel number also occurs.
Prior art attempts to reduce the effect of edge diffraction on the resonator mode have included the use of rounded edges, serrated edges, and zero equivalent length. All of these attempts have well known serious drawbacks.
Accordingly, what is still needed in the art, but as yet is not available, in some means for reducing the effect of edge diffraction on resonator modes, with the means also eliminating (or, at least, greatly reducing) any drawbacks.